I have an experience where I am on an UNLOCKED BlackBerry device using sim card from carrier "A".
I then downloaded and install official OS from BlackBerry website under carrier "B" name ie. OS 7.0.0.557_B.exe.
When i did the format/wipe my device with JL. CMD i forgot to delete the vendor.xml but it detect my device + new OS upgrade. And the upgrade installation run smoothly.
Desktop Manager might have gone out and found the latest OS version for your device's original carrier. The only thing the vendor.xml file does is make sure a Sprint device gets the official Sprint version of the OS, T-Mobile gets T-Mobile, etc. Other than that there is no use for the vendor.xml file. And since BlackBerrys are by nature completely carrier-agnostic, it's clearly only an attempt by the carriers to keep you using their officially sanctioned OS versions. Easier for them to offer support that way.